The present invention relates to a cathode, an electron beam apparatus, and a method of stabilizing the cathode used in electron beam application apparatus such as an electron beam lithography system and an electron microscope. Particularly, the present invention is related to shape of a tip of a cathode, a method of manufacturing the cathode, and a method of operating the cathode, which provide electron emission that is stable for a long period of time and uniform in teams of the energy among the electrons.
There has been practically used a Schottky emission cathode which has on a surface of its single crystal tip made of a refractory metal such as W and Mo, a metallic atom whose work function is lower than that of the single crystal tip, for example, Zr, Ti, or Hf and oxygen, of about one atomic thickness, respectively, adsorbed. U.S. Pat. No. 3,814,975 discloses that this emission cathode is fabricated by welding a single crystal piece with its tip sharpened by electrochemical etching on the top of a hairpin-shaped W filament, attaching hydride powder such as Zr hydride near the welded point, and heat-treating it in a vacuum atmosphere having a partial pressure of oxygen gas. A case that such a cathode is used at a high temperature of 1500 K. or more is particularly called a Schottky emission state. The basic structure of this cathode is shown in FIG. 1. Numeral 1 indicates a single crystal tip of W(100), 2 a hair pin type filament of W polycrystals, 4 terminals of stainless steel to which the filament 2 is spot-welded, and 5 a ceramic insulator. The cathode is structured so that a reservoir of oxide 3 such as Zr oxide, the work function of which is lower than that of the single crystal tip 1 of tungsten, is attached on the center or base of the single crystal tip 1 or on the filament 2. When it is heated at about 1500 K. to 1900 K., the oxide is thermally diffused along the single crystal tip 1. The metal oxide diffused toward the end of the single crystal tip 1 is adsorbed at the end of the single crystal tip 1 forms a layer of about one atom thickness of each of oxygen and the metal. The metal oxide is selectively adsorbed on a specific W(100) crystal surface having high activation energy of surface diffusion and desorption. When the W(100) crystal surface is formed at the end of the single crystal tip 1, only the end of the single crystal tip 1 can be kept in the state of a low work function. As a result, a high current density of electron emission can be obtained from that portion. Such a Schottky emission cathode, Zr/O/W is disclosed in Journal of Vacuum Science Technology, B3(1), 1985, p 220 et seq.
This cathode is characterized in that the energy width among emitted electrons is narrow and it can be operated continuously for several thousands hours unlike a normal thermionic emission cathode.
A method of processing an electron beam cathode for obtaining stable electron emission by this kind of cathode is disclosed in U.S. Pat. No. 4,324,999 and a method of stabilizing a cathode having lost stability in its operation is disclosed in Japanese Patent Application Laid-Open Hei 2-27643. These documents disclose the existence of a flat crystal face (hereinafter called a facet) at the tip of a cathode as a condition for obtaining stable electron emission. As a method of forming it, heating the cathode in oxygen is disclosed in U.S. Pat. No. 4,324,999 and applying a strong electric field on the cathode for a short time is disclosed in Japanese Patent Application Laid-Open Hei 2-27643.
Furthermore, operating conditions for this kind of cathode are disclosed in Japanese Patent Application Laid-Open Sho 60-501581. In this patent, it is stated that to prevent the tip of a cathode from blunting due to surface diffusion and to maintain current emission stably, it is necessary to apply an electric field over a certain value on the tip of the cathode and exert an electrostatic force acting toward the tip from the base.
Formulation of the balance between the surface diffusion and the attraction toward the tip from the base which is applied on the tip of a cathode by the electric field is disclosed in Physical Review, Volume 117, Number 6, p 1452 et seq. This paper reports experimental results on a simple W cathode, but not on a Schottky emission cathode. The methods described in U.S. Pat. No. 4,324,999 and Japanese Patent Application Laid-Open Hei 2-276643 require more than several hours for forming a facet, generally about 10 hours.